In a UK where legal narcotics were tightly regulated, would former dealers move into other areas of crime, or pack it all in?

Decriminalising drugs is a hot topic this week, with both Plaid Cymru and the Lib Dems publicly backing the idea to varying degrees. Granted, neither party is likely to be a frontrunner in the next election, and the Tories – anti-progression, anti-evidence and anti-logic when it comes to drug policy – are almost certainly going to sweep up.

However, the fact two mainstream parties are calling into question the effectiveness of the War on Drugs is a positive step nonetheless. In fact, even the tabloids and right-wing media are starting to see sense: The Daily Mirror has taken tentative steps towards supporting legalisation, and The Spectator published a pro-legalisation piece last month arguing that the move could help reduce gang-related crime.

This argument – that decriminalisation or legalisation would take away the main revenue stream of many career criminals – is often made by drug reform campaigners. But is it really that simple? Would legalising drugs result in crime rates suddenly dropping, or would dealers and traffickers just move into other areas of criminality? To find out I spoke to reformed London gangsters Leroy Smith and Paul Murdoch; the Bristol-based ex-villain Paul Scrase; former Scotland Yard detective David Videcette; and criminologist Dr Louise Shelley.

The consensus was that the majority of dealers would diversify into other illegal activities. Former drug trafficker and armed robber Smith thought gangsters might revert to carrying out hold-ups, one of the main forms of organised crime before drugs became popular. This reminded me of something Mancunian career criminal Colin Blaney had told me while I was ghostwriting his memoir: that when crack and heroin first hit, all the old armed robbers realised they could make easier money selling hard drugs, so made an overnight transition from blaggers to dealers.

If all drugs were legalised, would this transition simply be reversed?

Videcette doesn't think so. "I don't think we'd ever see large-scale robbery of banks or cash in transit like we used to, primarily because we're becoming a cashless society and everything is online, so the future looks more like non-violent thefts of virtual money and fraudulent transfers," he says. "What will drug dealers do if there's no profit left for them in drugs? They'll most likely drift across into something not too far away from their current skill set, so traffickers will traffic other things, such as guns, people and drugs stolen from legal supply routes."

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According to Shelley, drug organisations are already diversifying into people smuggling. In May of last year, former UK Border Agency head Tony Smith reported a major increase in people trafficking gangs, and warned that they "are making so much cash they now rival the drugs trade". It seems plausible, then, that if drugs were legalised, other groups that previously stuck to importing and exporting narcotics might decide they want a piece of the pie.

Scrase says that dealers might start selling drugs legally, but use the threat of violence to ward off industry competitors. "You're going to have rivals trying to knock out the competition," he tells me. "Prices would probably hit rock bottom, but that would just cause more problems."

Of course, this scenario would only occur if few restrictions were placed on who can set up a legal drug selling business, and if drugs were ever legalised the trade would be tightly regulated. However, Smith reckons this would almost certainly result in a thriving black market. When prohibition was abolished in the US there were still problems with bootleggers, as hard liquor distilling had relatively restrictive limits placed upon it. Similarly, in New York, the current limitations on the sale of medical marijuana mean that people with genuine illnesses still frequently purchase it illegally. So it stands to reason that the same would happen with all other narcotics, whether it's because of cost, quality or accessibility.

On that last point, Videcette points out that as well as there being restrictions on who could sell drugs, there will also be people unable to buy them via the legal channels. "There will always be those who do not wish to – or cannot – use legalised drugs from government-registered suppliers," he says. "Professionals who use drugs, for example – such as doctors, police officers, lawyers and the like – may never want their names on official lists of users because of the impact it might have on their careers, and they will always use black market suppliers."

"No one's a gangster for the sake of it; it's all because of the need to make money, so if they can make it legally, they'll do that instead."

Videcette also predicts a thriving market in stolen drugs. "Organised criminals would simply switch to other profitable forms of acquisition of the drugs, such as theft from the supply chain, or theft and sale from registered suppliers. So they will still sell drugs, but their methods of acquisition will diversify," he says. "A look at US states that have legalised cannabis shows us that organised criminals are now using government-registered supply networks to increase cannabis usage in places where cannabis isn't legal, buying it in legalised states and then selling it at huge profit in states where it's illegal."

Given that you can hop on a ferry in Dover and be in Calais in an hour-and-a-half, were drugs legalised it seems likely that former dealers might buy huge amounts of them in the UK and smuggle them across to mainland Europe. Yes, it's harder to sneak something overseas than it is to traffic it across state lines, but important barriers are removed within the UK.

So one takeaway is: criminals will always do crime. Mind you, a couple of the former gangsters agree that a small number of dealers might pack it in altogether and take up legal professions instead. Murdoch claims that some dealers would lack the skills or inclination to make the transition to another illegal activity. "Some people who sell drugs just do it because they know people they can buy them from and are in the right place at the right time," he says. "They wouldn't be able to do other things. You'd also get a few who'd think, 'You know what? I don't like doing armed robberies and I don't like tying people up,' and wouldn't do it."

According to Smith, those only suited to drug dealing might plough their ill-gotten gains into legitimate businesses instead. "They'd go into anything that brings in lots of money legally," he says. "No one's a gangster for the sake of it; it's all because of the need to make money, so if they can make it legally, they'll do that instead."

Of course, it's impossible to know for certain what effect the complete legalisation of all drugs would have. Decriminalisation in Portugal has had a largely positive impact, but there's a difference between stopping the prosecution of drug users and allowing people to freely import, buy and sell everything from coke to heroin.

Perhaps one day we'll find out what ramifications total legalisation would have, but until a nation is brave enough to take the plunge we can only speculate.